1. Field of the Invention
The present invention relates to a directional coupler, and more particularly, to a directional coupler which includes a main line and a sub-line that are electromagnetically coupled to each other.
2. Description of the Related Art
For example, a known directional coupler is disclosed in Japanese Patent No. 3203253. In the directional coupler, a first coupling line in a spiral or substantially spiral shape faces a second coupling line in the same shape as the first coupling line with a dielectric layer therebetween. With this configuration, the first coupling line and the second coupling line are electromagnetically coupled to each other and form a directional coupler.
With the directional coupler described in Japanese Patent No. 3203253, in order to make a fine adjustment of the degree of coupling between the first coupling line and the second coupling line, adjusting the thickness of the dielectric layer provided between the first coupling line and the second coupling line is considered. However, since the first coupling line is provided on one dielectric layer and the second coupling line is provided on another dielectric layer, adjusting the thickness of the dielectric layer provided between the first coupling line and the second coupling line causes the entire first coupling line and the entire second coupling line to be closer to or farther away from each other. Therefore, the degree of coupling between the first coupling line and the second coupling line will greatly vary. As described above, it is difficult for the directional coupler described in Japanese Patent No. 3203253 to make a fine adjustment of the degree of coupling between the first coupling line and the second coupling line.